1. Technical Field
The present invention relates to a signal receiver and, more specifically, relates to a signal receiver using a burst detector to detect the occurrence of a burst.
2. Description of the Related Art
A pulse communication receiver, such as a digital receiver or a radar receiver, must obtain a time reference to decode a received signal. A burst can be detected in the received signal to provide the time reference. In a digital communication system, such as a TDMA (time division multiple access) communication system., frames of information are periodically received. A timing reference for a received frame can be obtained by detecting any expected burst at a deterministic position within the frame. For example, a burst occurring at the beginning or other location of a frame can be detected to obtain a time reference for decoding the received signal. Once a burst has been detected information can be extracted from the frame or other portions of the received signal. This information can also be used to obtain timing for subsequent frames. Such frame synchronization is required before detecting information to provide an output for the user of the receiver.
In a previous receiver, a received signal is correlated with an expected pattern to establish a timing reference. Specifically, the correlation of the received signal with the expected signal is followed by detection of a correlation peak to establish the timing reference. This system requires transmission from a transmitter to a receiver of dedicated patterns consuming valuable frequency spectrum and restricting system capacity. Should a system be established without dedicated patterns for establishing a timing reference, system capacity is increased and frequency spectrum conserved.
When the transmitter and receiver obtain large frequency differences, the above correlation technique becomes unreliable. These large frequency differences can be caused by differences in the transmitter's and receiver's reference frequencies due to, for example, crystal errors. Furthermore, this large frequency difference can be caused when the receiver moves relative to the transmitter at a large velocity. For example, an aircraft or a satellite is fast moving and typically would have Doppler frequency errors when communicating with a ground station or another aircraft or satellite. As the transmitter and receiver obtain a larger frequency difference, the received signal moves outside the range of correlation with the expected pattern. Thus, as the frequency difference increases, the received signal and expected pattern become increasingly decorrelated and hence more difficult to establish a timing reference.
In another known receiver, such as a Rake receiver, multiple receiver paths each having a different frequency offset perform simultaneous correlation with an expected pattern to establish a time reference. As a result of having multiple receiver paths, the frequency difference seen by one of the receiver paths may be small enough to get an adequate detection of a correlation peak. However, this approach requires multiple receiver paths adding additional cost and complexity to the receiver. Furthermore, the multiple receiver paths require additional processing time and could cause delays before a choice between the multiple paths can be made.
The performance of either of the above techniques also degrades as the signal to noise ratio decreases. This performance degradation is caused by false detection of the correlation peak. As the signal to noise ratio decreases, correlation peaks due to noise are hard to distinguish from a correlation peak with the expected pattern.